Pebble results as a by-product from phosphate rock mining operations. In Northern Florida the pebble is of high alumina, iron and magnesia content and is considered a waste material which presented a disposal problem. The pebble has a size ranging from 1/4" to +14 mesh and a BPL (Bone Phosphate of Lime) value ranging from 38 to about 68 (17.8-31.2% P.sub.2 O.sub.5). With the declining quality of rock, in recent years small amounts of pebble have been ground and blended into higher quality, beneficiated rock. In the past, pebble having a BPL value higher than 58 (26.6% P.sub.2 O.sub.5) was stored while the lower grade, high alumina pebble was discarded with the tailings.
Typically, pebble is obtained by screening "as mined" rock and retaining the +14 mesh or "pebble" fraction, but in recent years some pebble has been retained from screening at +10 mesh.
In spite of the tremendous reserve of P.sub.2 O.sub.5 which the high alumina pebble represents, it was not processed because of its high content of metal compounds such as compounds of calcium, magnesium, aluminum and iron. This is due in part to a need to produce phosphoric acid of low metallic ion content that can be readily converted to superphosphoric acid (70-76% P.sub.2 O.sub.5) for use in liquid fertilizer manufacture.
When high alumina phosphate pebble rock is digested with phosphoric acid and sulfuric acid to form aqueous phosphoric acid and calcium sulfate by the usual gypsum processes, a great many soluble metallic phosphates are formed. When the phosphoric acid is concentrated to merchant grade phosphoric acid and superphosphoric acid, after removal of the solid calcium sulfate and gangue and after clarification, the presence of the soluble metallic phosphates values greatly increases the viscosity of the phosphoric acid. The viscosity of the phosphoric acid can increase to such an extent that the phosphoric acid cannot be handled or transported as a fluid. As the concentration of phosphoric acid is increased, the solubility of the metallic phosphates sharply decreases to form sludges, scale and complex precipitates in phosphoric acid. These sludges settle out in the phosphoric acid creating storage, handling and transportation problems for the acid. In addition, formation of sludges, scale and precipitates causes an appreciable loss of the P.sub.2 O.sub.5 values in the phosphoric acid.
As can be seen in the pebble analyses given in the examples herein, the total of the analyses of Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 in high alumina pebble can be at least about 3.2 weight % (in Example 4) and at least about 3.78% (in Example 1). The total of the analyses for Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3 and MgO can be at least about 3.73 (in Example 4) and at least about 4.47 (in Example 1). The weight % CaO can be about 44.1 (in Example 4) and about 43% (in Example 1).
The ratio of each impurity (e.g., metals, SiO.sub.2, F, carbonate) to P.sub.2 O.sub.5 is also indicative of the difficulty of economically manufacturing wet process phosphoric acid from a a given phosphate rock. In general, the CaO/P.sub.2 O.sub.5 ratio will tell whether the sulfuric acid consumption will be too costly. That is, any calcium in excess of stoichiometric for the so-called "marine apatite" or fluorapetite (e.g., Durango) can be considered undesirable because it can consume sulfuric acid without producing the stoichiometric quantity of phosphoric acid.
When the high alumina pebble is initially converted to impure phosphoric by a hemihydrate process, in the order of 50% less alumina is extracted from the rock into the acid than is extracted by the usual gypsum process which makes 22 to 30% P.sub.2 O.sub.5 acid V/S at least 36% P.sub.2 O.sub.5 by hemihydrate processes.
Other factors which made pebble use prohibitive by the gypsum-type wet process is the higher sulfuric acid requirements per ton of pebble, as compared to phosphate rock, to react with and precipitate calcium. This is another advantage which can arise from use of such hemihydrate processes as those of the patents of Ore' and of Ore', Ellis and Moore; namely, lower sulfuric acid consumption.
The following table compares typical phosphate rock and high alumina pebble produced in Northern Florida:
______________________________________ Amount (Weight-Percent) Values Phosphate Rock High Alumina Pebble ______________________________________ P.sub.2 O.sub.5 34.32 27.46 INSOL 4.5 7.80 CaO 48.4 39.90 MgO 0.3 0.76 Al.sub.2 O.sub.3 1.5 2.10 Fe.sub.2 O.sub.3 0.6 1.30 F 3.75 3.30 SiO.sub.2 3.0 9.90 CO.sub.2 3.75 6.00 Organic Carbon 0.15 0.20 ______________________________________
The present process can produce low impurity wet process phosphoric acid from such high alumina pebble especially uses the hemihydrate digestion steps.
In Central Florida, the pebble is coarser, generally has a BPL value around 68 (31% P.sub.2 O.sub.5) and is of lower metallic content than Northern Florida pebble. In addition, it is sand-free and clay lumps disintegrate easily, which aids in its removal. This pebble has a market value because it may be used as a feed to electric furnaces to produce elemental phosphorous and can also be used with phosphate rock within restrictions imposed by product quality standards.
However, high alumina pebble inventory in Central Florida also increased with increased emphasis on low metallic content phosphoric acid which can be converted to high quality superphosphoric acid. This made even more remote the processing of high alumina pebble by the usual gypsum type wet process, in both Central and certainly Northern Florida.
Beneficiated phosphate rock also contains metallic impurities, though less than a high alumina pebble, which are undesirable and complicate the production of superphosphoric acid due to the metallic phosphate complexes that are formed which increase the viscosity of the acid and form sludges, which settle resulting in a P.sub.2 O.sub.5 loss.
"Pebble or phosphate pebble," as used hereinafter, refers to the phosphate pebble rock, particularly high alumina phosphate pebble rock which is a by-product in phosphate mining operations.
"Pebble acid," as used hereinafter, refers to the phosphoric acid which is made from phosphate pebble rock.
"Sulfonic" acid and "sulphonic" acid are used interchangeably in the specification and have the same meaning.